In a typical wireless communication network, also known as radio communications network, wireless terminals, also known as mobile stations and/or user equipments (UEs), communicate via a Radio Access Network (RAN) to one or more Core Networks (CN). The RAN covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g., a Radio Base Station (RBS), which in some networks may also be called, for example, a NodeB or evolved Node B (eNodeB, eNB). A cell is a geographical area where radio coverage is provided by the radio base station at a base station site or an antenna site in case the antenna and the radio base station are not collocated. Each cell is identified by an identity within the local radio area, which is broadcast in the cell. Another identity identifying the cell uniquely in the whole mobile network is also broadcasted in the cell. One base station may have one or more cells. A cell may be a downlink cell and/or an uplink cell. The base stations communicate, transmit signals and/or receive signals, over the air interface operating on radio frequencies with the wireless devices within range of the base stations.
A Universal Mobile Telecommunications System (UMTS) is a third generation mobile communication system, which evolved from the second generation (2G) Global System for Mobile communications (GSM). The UMTS Terrestrial Radio Access Network (UTRAN) is essentially a RAN using Wideband Code Division Multiple Access (WCDMA) and/or High Speed Packet Access (HSPA) for wireless devices. In a forum known as the Third Generation Partnership Project (3GPP), telecommunications suppliers propose and agree upon standards for third generation (3G) networks and UTRAN specifically, and investigate enhanced data rate and radio capacity. In some versions of the RAN as e.g. in UMTS, several base stations may be connected, e.g., by landlines or microwave, to a controller node, such as a Radio Network Controller (RNC) or a Base Station Controller (BSC), which supervises and coordinates various activities of the plural base stations connected thereto. The RNCs are typically connected to one or more core networks.
Specifications for the Evolved Packet System (EPS) have been completed within the 3GPP and this work continues in the coming 3GPP releases. The EPS comprises the Evolved Universal Terrestrial Radio Access Network (E-UTRAN), also known as the Long Term Evolution (LTE) radio access, and the Evolved Packet Core (EPC), also known as System Architecture Evolution (SAE) core network. E-UTRAN/LTE is a variant of a 3GPP radio access technology wherein the radio base stations are directly connected to the EPC core network rather than to RNCs. In general, in E-UTRAN/LTE the functions of a RNC are distributed between the radio base stations, e.g. eNodeBs in LTE, and the core network. As such, the RAN of an EPS has an essentially “flat” architecture comprising radio base stations without reporting to RNCs.
In GSM and many other transmission systems, the transmitted signal consists of bursts of high energy, separated by periods with low or no energy, so called guard periods. The transmitted information is represented in the high energy part using a digital modulation such as Gaussian Minimum Shift Keying (MSK), Binary Phase-Shift Keying (BPSK), Quadrature Phase-Shift Keying (QPSK), 8 Phase-Shift Keying (8PSK), 16-state Quadrature Amplitude Modulation (16QAM), etc. In a baseband representation, a burst can be described by a trajectory in the complex, In-phase/Quadrature (I/Q) plane, starting and ending at, or close to, origin at the start and end of the burst, respectively. The trajectory as a function of time represents the information bits that are transferred by the burst. For example, the phase, the amplitude, the frequency or combinations thereof can be used to convey the information.
When a burst is transmitted over a channel, the amplitude, phase and/or timing of the burst may be impacted in a way that is not known a priori to the transmitter or receiver. Therefore, in coherent wireless communications systems, the information is not carried in the absolute phase and/or amplitude of the transmitted signal, but rather the phase/amplitude relative to a predetermined part of the burst, known to both the transmitter and the second receiver. The known part is sometimes referred to as training symbols or pilot symbols.
On many channels, for instance radio channels, the signal is also subject to time dispersion. The way the channel causes time dispersion must be known to the second receiver in order for it to compensate for the dispersion to correctly demodulate it. To facilitate this, the bursts in many transmission systems contain a sequence of known symbols, called e.g. training sequence, preamble or midamble. This is used both to estimate the time dispersion of the channel, and the time/phase/amplitude reference. Due to this, an entire transmitted burst can be amplified/attenuated, phase shifted and/or time shifted, within reasonable limits, without changing the information carried by the burst. Hence, the transmission link must be robust enough to handle such modifications.
There are transmission modes where the absolute phase of the transmitted signal need to be well defined, for example when applying spatial beam forming, or when the signal needs to follow a relative phase shift in-between transmissions, for example when applying space time codes. This is however generally not the case, and is especially not required for GSM.